<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        .left{dom布局写为左右中
            width: 200px;
            height:1200px;
            float: left;
            background-color: yellow;
        }
        .right{
            width: 300px;
            height: 1200px;
            float: right;
            background-color: pink;
        }
        .main{
            height: 1200px;
            background-color: green;
            margin-left: 200px;
            margin-right: 300px;
            /*float: left;*/
        }
    </style>
</head>
<body>
    <div class="left">左侧</div>
    <div class="right">右侧</div>
    <div class="main">中间栏目</div>
</body>
</html>